Selective science in road risk: energy and speed cameras

Many safety issues are bolstered with facts and figures and scientific terms designed to convince the reader of their merit.

An example is this from the Department of Justice in Victoria, Australia that has produced a web page with the domain camerassavelives in what seems at least in part to be a quest to quell skepticism about the purpose of speed cameras.

If you doubted the merits of speed cameras perhaps an impressive-sounding section called ‘Crash Physics‘ will convince you.  You can’t argue with physics. Right?

The Commission writes that kinetic energy causes human injuries or death in a crash”.  Ok, kinetic energy.  Tell us more.

The Department helpfully inform us that this dangerous phenomenon increases “rapidly” as speed increases.  Seemingly if those speed cameras can keep the lid on kinetic energy they sure will “save lives”.  Physics proves it.  No more arguments there.

But if it is kinetic energy that “causes” human injuries, why are we just focused on the speed?  KE= 0.5 x mass x velocity squared.  So the kinetic energy of a moving object is proportional to the square of the velocity, as it seems the Department was suggesting, but also in proportion to the mass which they do not mention.

So a car driver in a 2000kg car doing 105km/hr will have their life “saved” by a benevolent speed camera, while a 50t truck coming the other way at 100km/hr passes by even though the truck has 23 times as much kinetic energy as the “bad driver’s” car.  What?  I thought kinetic energy was the cause of “human injuries or death in a crash”.

Car, weight=2000kg, speed=105km/hr, kinetic energy (the stuff that causes injury)=0.85MJ

Truck, weight=50,000kg, speed=100km/hr, kinetic energy=19.3MJ

“Kinetic energy” causes death and injury according to the Department, yet the “life-saving” camera gives a ticket to the car even though when speeding it has only 4% of the death-causing energy of the truck.

You can’t make sense of that.  It’s impossible.

What is happening here appears to be that decisions taken for one reason are justified with snippets of science.  Elements of truth.  None of it untrue.  But not the full truth either; just enough to sell the story.  If you put it to the test it fails.  The science of energy is not the real reason for speed cameras.  It can’t be.  What we see here presents therefore as the marketing of a predetermined plan masquerading with a cover of selective science.

Ultimately there is little point using science for this purpose.  It’s not credible.  If the surface is scratched the argument crumbles. Just say the truth instead.  Something like:

We are are not really sure about the plan to (insert name of safety initiative) but we are doing it because we “reckon it’s a good idea”.


About John Culvenor

Hi, Thank you for taking a look at this blog. I work in engineering, ergonomics, creativity, design, training, etc. Often this is about helping solve legal puzzles through accident analysis. Sometimes it is about thinking up better designs for equipment, workplaces, and systems. This blog is about good design and bad design, accident analysis and how it can be done better, and how we can make a better, safer world by design!
This entry was posted in Uncategorized. Bookmark the permalink.

3 Responses to Selective science in road risk: energy and speed cameras

  1. Pingback: “I swear to tell the truth, part of the truth, the convenient part of the truth”. The selling of safety in Australia. | safedesign

  2. John Ballantyne says:

    I take your point, but do you think maybe you are being a bit hard on the people who are trying to make the point that “speed kills” with “science”?

    The fact that kinetic energy is proportional to the square of speed is an inescapable determinant in the physics of collisions, their causes and their outcomes. Sure, there is the kinetic energy of the vehicle, and you point out that massive trucks have a lot more than cars, but you must also consider the kinetic energy in the bodies of the occupants, which is also proportional to the square of speed. The collision that kills is between THOSE bodies and the surrounding solid masses that came to such a sudden stop.

    Another issue is that STOPPING DISTANCE increases with the square of speed, and that is as true for a truck as it is for a sub-compact. It is much harder for any driver, truck or car, to avoid a collision as their speed increases.

    Some people like to point out that it is the differential speed between vehicles on a highway that counts, but this is only partly true.

    When people discuss the problem of speeding, they are often thinking about expressways, and speeds in excess of 100 km/hr. In fact, the speed that kills is in places like school zones and intersections, where if drivers had driven at posted speeds, they would have had the time to avoid the collision.

    We are talking about life and death here, so even though I agree with you that the “science” is sometimes oversimplified by safety advocates, I think that any and every message aimed at making people pay attention to their speed is a good message.

  3. Hi John,

    Thank you for the thoughtful contribution.

    Taking your idea on board (energy of occupants), the Department of Justice don’t say that it’s the people inside the vehicle who’s energy is a concern, but if that’s what they mean, then we arrive at the same problem. If we really have a human ke limit on the road instead of a speed limit, then lighter people should be allowed to drive faster.

    Just what “crash physics” is the Department of Justice they talking about? This is a government department. If the government is talking about physics shouldn’t it be clear what they mean and exactly right?

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s